1. Field of the Invention
The present invention relates to a display device for converting an analog video signal outputted from a personal computer or a workstation into a digital video signal to display the digital video signal. More particularly, the invention relates to a technique for automatically adjusting image quality on a screen to an optimum level. The technique of the present invention is satisfactorily applicable to flat display devices capable of displaying a digital image which are represented, for example, by a liquid crystal display device and a plasma display device.
2. Description of the Background Art
FIG. 9 is a block diagram of a prior art liquid crystal display device disclosed in Japanese Patent Application Laid-Open No. P07-219485A (1995). The device of FIG. 9 samples an output video signal from a personal computer and the like to display an image on a liquid crystal display panel. The operation of the device is described below.
Referring to FIG. 9, pixel data for at least a predetermined part within a first frame are sampled at a fixed phase and then stored in a first memory. A frame which appears n frames later than the first frame is defined as a second frame. Pixel data for the same part within the second frame as within the first frame are sampled at the abovementioned phase. A comparator circuit compares the sampled pixel data with the pixel data of the first frame to judge whether or not both of the pixel data substantially coincide with each other for each horizontal scanning line. A controller counts the number of horizontal scanning lines in which the substantial coincidence is detected, based on the result of comparison outputted from the comparator circuit. The controller repeats the above described processing, with the sampling phase varied, to store the count and the sampling phase at that time in a second memory for each cycle of the processing. The controller compares the results stored in the second memory to extract a sampling phase at the time of the greatest number of horizontal scanning lines in which the pixel data for the predetermined part after the n frames substantially coincide with the pixel data for the predetermined part within the first frame. Then, the controller uses the extracted sampling phase as an optimum phase of a sampling clock to display a video signal. This allows the sampling phase to be automatically set near the center of pixels to execute appropriate sampling.
Unfortunately, the prior art liquid crystal display device is required to manually adjust a video signal capturing position prior to the judgement of the phase coincidence.
Additionally, the prior art liquid crystal display device adopts a scheme for comparing the pixel data for the predetermined part of a given frame with the pixel data corresponding to the predetermined part after the n frames as a means for judging the phase coincidence. This scheme finds difficulties in detecting the optimum phase if noises are superimposed on an input video signal or if the personal computer provides unstable outputs.
Further, the prior art liquid crystal display device which gives no consideration to periodical automatic adjustment is not capable of coping with changes, if any, in the phase of the sampling clock due to a temperature drift in the personal computer or in a sampling clock generating circuit of the device. Even when the prior art liquid crystal display device periodically makes an automatic adjustment, the sampling phase is changed every n frames during the phase adjustment so far as the above described scheme is employed. This is disadvantageous in that normal pictures are not outputted on the screen during the adjustment.
The prior art liquid crystal display device presents another problem in that the use of n-frame video data for each measurement of the number of scanning lines requires much processing time to make a final judgement.
Such problems are not unique to the liquid crystal display device illustrated in FIG. 9 but are encountered also by other devices which employ a similar scheme to convert an analog video signal into a digital video signal to display a digital image.